The present invention is related to an improved polymerization method for preparing solid electrolytic capacitors. More specifically, the present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. Even more specifically, the present invention is related to a capacitor comprising improved crosslinking within the conductive polymeric cathode layer thereby improving adhesion as evidenced by improved ESR and ESR stability.
Electroconductive polymers are widely used in capacitors, solar cells and LED displays. The electroconductive polymers include polypyrrole, polythiophene and polyaniline. Among them, the most commercially successful conductive polymer is poly(3,4-ethylenedioxy thiophene) (PEDOT). One way to apply PEDOT is by forming the PEDOT polymer via in-situ chemical or electrochemical polymerization. The other way is to use it as a PEDOT dispersion preferably with a polyanion, which has much better solubility than PEDOT itself. More particularly, PEDOT-polystyrene sulfonic acid (PEDOT-PSSA) dispersion has gained a lot of attention due to its high conductivity and good film forming property.
Today, almost all electronic components are mounted to the surface of circuit boards by means of infra-red (IR) or convection heating of both the board and the components to temperatures sufficient to reflow the solder paste applied between copper pads on the circuit board and the solderable terminations of the surface mount technology (SMT) components. A consequence of surface-mount technology is that each SMT component on the circuit board is exposed to soldering temperatures that commonly dwell above 180° C. for close to a minute, typically exceeding 230° C., and often peaking above 250° C. If the materials used in the construction of capacitors are vulnerable to such high temperatures, it is not unusual to see significant positive shifts in ESR leading to negative shifts in circuit performance. SMT reflow soldering is a significant driving force behind the need for capacitors having temperature-stable ESR.
Equivalent Series Resistance (ESR) stability of the capacitors requires that the interface between the cathode layer, cathodic conductive layers, conductive adhesive, and leadframe have good mechanical integrity during thermo mechanical stresses. Solid electrolytic capacitors are subject to various thermomechanical stresses during assembly, molding, board mount reflow, etc. During board mount the capacitors are often subjected to temperatures above 250° C. These elevated temperatures create stresses in the interfaces due to coefficient of thermal expansion (CTE) mismatches between adjacent layers. The resultant stress causes mechanical weakening at the interfaces. In some cases this mechanical weakening causes delamination. Any physical separation of the interfaces causes increases in electrical resistance between the layers and thus an increased ESR in the finished capacitor.
PEDOT-PSSA polymer film often does not have enough mechanical strength or sufficient adhesion to the underlying surface. In capacitors, poor film quality and adhesion results in poor ESR or poor ESR stability under processing conditions. Polymeric binders can be added to enhance the mechanical properties of the PEDT-PSSA film and adhesion to the anode. In U.S. Pat. No. 6,987,663, which is incorporated herein by reference, the conductive polymer coating included at least one polymeric organic binder. In U.S. Pat. No. 7,990,684, which is incorporated herein by reference, the conductive polymer coating contains a Novolak polymer resin and a sulfonated polyester as binders.
The polymer binder may be formed “in situ” during the drying step as described in U.S. Published Patent Application 2012/0256117, which is incorporated herein by reference, wherein described is a polymer dispersion of PEDOT-PSSA comprising a polyhydric alcohol and an organic substance having two or more functional groups which can be polycondensed with the polyhyric alcohol to form a polymer binder “in situ”.
Another problem associated with PEDOT-Polyanion, especially PEDOT-PSSA conductive polymer film is the hydroscopic property of the polyanions. Polyanions readily absorb water during the capacitor processing steps (for example, dipping coating cycles) or moisture from the environment, and resulted in swelling of the conductive polymer film. The swollen conductive film is typically subjected to drying steps later on. The swelling/shrinking cycles often cause the conductive polymer film to delaminate from the substrate. In capacitor application, it is manifested as deteriorated performance such as positive ESR shift.
EP 0844284, which is incorporated herein by reference, describes a conductive polymer self-doped by —SOOH and/or —COOH functional groups wherein the self-doping groups are on the conductive polymer structure. An advantage of using self-doped conductive polymer over external doped polymer as in the case of PEDT-polyanion dispersion, is the elimination of polyanions which are detrimental to moisture resistance. Still, these self-doped polymer films have poor water or solvent resisting properties. The conductive polymer film's water resistance property can be improved by reacting the self doping groups —SOOH or —COOH with a crosslinking compound having 2 or more functional groups such as a hydroxyl, a silanol, a thiol, an amino or an epoxy group.
For more hydroscopic externally doped PEDOT-PSSA, U.S. Published Patent Application 2010/0091432, which is incorporated herein by reference, described the use of organic substance with a mono epoxy group in PEDOT-PSSA to improve its water resistance. In comparison, an epoxy compound having multiple epoxy groups in the conductive polymer composition resulted in inferior water resistance property and higher ESR.
In spite of the ongoing efforts there is still a significant problem associated with coating stability in electrolytic capacitors utilizing conductive polymer cathodes. Further advances in the art are provided herein.